Variadic Templates: Unraveling Pack Expansion for Iteration
Variadic templates grant remarkable expressiveness in C , but venturing into their realm can reveal unforeseen complexities. One such challenge arises when attempting to iterate over a parameter pack using pack expansion. Consider the following code:
template<typename t> static void bar(T t) {} template<typename... args> static void foo2(Args... args) { (bar(args)...); } int main() { foo2(1, 2, 3, "3"); return 0; }</typename...></typename>
Upon compilation, this code encounters an error: "'args': parameter pack must be expanded in this context." The issue stems from the inability to directly expand a parameter pack in a function call.
The solution lies in utilizing pack expansion within a braced-init-list, a context that explicitly allows such expansion. By placing the expansion inside the initializer list of a dummy array, we can force the desired behavior:
template<typename... args> static void foo2(Args &&... args) { int dummy[] = { 0, ( (void) bar(std::forward<args>(args)), 0) ... }; }</args></typename...>
This revised code embraces a meticulous approach to ensure correct evaluation:
- Guard Against Empty Expansion: An empty Args will result in a 0-length array, which is illegal in C . To prevent this, we ensure the array contains at least one element.
- Perfect Forwarding: We use perfect forwarding (std::forward) to avoid potential copies and preserve potentially-const qualifiers.
- Neutralize Return Type: We cast bar()'s return value to void to leverage the built-in comma operator, ensuring left-to-right evaluation.
In C 17, fold expressions provide an even more compact solution:
((void) bar(std::forward<args>(args)), ...); ``` This approach guarantees the desired left-to-right expansion unequivocally. </args>
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